1. Field of the Invention
The present invention relates to a magnetic recording medium which is employed in, for example, hard disk devices, to a process for producing the magnetic recording medium, and to a magnetic recording and reproducing apparatus.
2. Background Art
The recording density of a hard disk device (HDD), which is a magnetic recording and reproducing apparatus, has increased at a rate of 60% per year, and this tendency is expected to continue. Therefore, magnetic recording heads and magnetic recording media which are suitable for attaining high recording density are now under development.
Magnetic recording media employed in hard disk devices are required to have high recording density, and therefore demand has arisen for improvement of coercive force and reduction of medium noise.
Most magnetic recording media employed in hard disk devices have a structure including a magnetic recording medium substrate on which a metallic film is laminated through sputtering. Aluminum substrates and glass substrates are widely employed for producing magnetic recording media. An aluminum substrate is produced through the following process: an Ni—P-based alloy film (thickness: about 10 μm) is formed through electroless plating on an Al—Mg alloy substrate which has undergone mirror polishing, and the surface of the Ni—P-based alloy film is subjected to mirror polishing. Glass substrates are classified into two types; i.e., amorphous glass substrates and glass ceramic substrates. When either of these two types of glass substrate is employed to produce a magnetic recording medium, the substrate is subjected to mirror polishing.
In general, a magnetic recording medium employed to produce a hard disk device includes a non-magnetic substrate; a non-magnetic undercoat layer (formed of, for example, an Ni—Al-based alloy, Cr, or a Cr-based alloy); a non-magnetic intermediate layer (formed of, for example, a Co—Cr-based alloy or a Co—Cr—Ta-based alloy); a magnetic layer (formed of, for example, a Co—Cr—Pt—Ta-based alloy or a Co—Cr—Pt—B-based alloy); a protective film (formed of, for example, carbon), the layers and film being successively formed on the substrate; and a lubrication film containing a liquid lubricant and formed on the protective film.
In accordance with an increase in recording density of, for example, a magnetic disk device, demand has arisen for a magnetic recording medium exhibiting magnetic anisotropy in a circumferential direction (hereinafter may be referred to simply as “circumferential magnetic anisotropy”), and thus exhibiting excellent read-write conversion characteristics. Therefore, circumferential magnetic anisotropy is imparted to a magnetic recording medium including an aluminum alloy substrate coated with an NiP film through plating (hereinafter the substrate may be referred to as an “aluminum substrate”), by means of mechanically forming grooves on the surface of the NiP film in a circumferential direction (hereinafter the procedure will be referred to as “mechanical texturing”).
Non-magnetic substrates; for example, glass substrates, are suitable for attaining high recording density, since they exhibit excellent impact resistance, rigidity, and evenness. Therefore, when a magnetic recording medium including a non-magnetic substrate formed of glass can be provided with circumferential magnetic anisotropy, the resultant recording medium is envisaged to exhibit excellent read-write conversion characteristics.
Several methods have been proposed for subjecting a glass substrate to mechanical texturing, to thereby form texture grooves thereon. For example, there has been proposed a method for forming minute, uniform texture grooves, which employs a suspension containing abrasive grains and a solution of a hydroxyl-group-containing compound, and a woven tape formed of plastic fiber. See Japanese Patent No. 3117438.
There has also been proposed a method for forming minute, uniform texture grooves, which employs diamond abrasive grains together with CeO2 abrasive grains. See U.S. Pat. No. 6,248,395.
However, difficulty is encountered in imparting sufficient circumferential magnetic anisotropy to a magnetic recording medium including a glass substrate by merely forming texture grooves on the substrate. In view of the foregoing, there has been proposed a method in which an amorphous layer containing at least Ni and P is formed through sputtering on a glass substrate having linear texture grooves on the surface thereof, to thereby impart circumferential magnetic anisotropy to the resultant magnetic recording medium. See Japanese Patent Application Laid-Open (kokai) No. 2001-209927.
The above-proposed method for forming an amorphous layer containing at least Ni and P on a glass substrate having texture grooves constitutes an attempt to obtain an effect similar to that obtained by the aforementioned NiP-plated aluminum substrate having texture grooves. In reality, when a Cr-based undercoat film, a Co-based magnetic layer, and a protective film are successively formed on the amorphous layer formed through the above method, the resultant magnetic recording medium exhibits magnetic anisotropy. However, when the amorphous layer containing at least Ni and P is employed, difficulty is encountered in attaining high coercive force and high squareness ratio, and satisfactory read-write conversion characteristics fail to be obtained.